During the drawing of optical fibers both surface and volume defects are generated in the optical fiber.
In the former case the defects consist in superficial micro-fractures of different depth which according to their type and density affect more or less seriously the mechanical properties of resistance to traction and dynamic fatigue of the fiber.
In the latter case they consist in lattice defects such as micro-crystals, bond defects, etc, which give rise to the formation of color centers, i.e. zones in which the optical radiation is absorbed according to a law dependent on the wavelength. These are generally gaussian curves with a maximum in the ultraviolet band. In addition, the volume defects referred to above raise the value of the Rayleigh scattering coefficient, and hence the entire attenuation curve. Finally, volume defects increase the network reactivity with polluting agents, such as molecular hydrogen, etc, which can diffuse inside giving rise to new absorption zones.
In Proc. Optical Fiber Communication Washington D.C. Mar. 6-8, 1979, pages 74, 75 the paper entitled "Improved fatigue resistance of high-strenght optical fibers" by R. Hiskes, describes an attempt at eliminating at least surface defects by directly applying on the fiber, during drawing and prior to coating it with resin, a thin layer of ceramic material.
However surface defects are not completely avoided, as the process cannot take place at the high temperatures, at which fiber deformations might take place and hence the adherence between the applied ceramic layer and the fiber is poor. Besides, since the deposition is made directly on the fiber, the drawing speed is slowed down and manufacturing times are increased.
These disadvantages are overcome by a method of reducing volume and surface defects in silica optical fibers, provided by the present invention, by which surface defects of vitreous silica fibers can be avoided by enriching the surface with a compound which changes its thermomechanical properties preventing micro-fractures, without perturbations in the chemical equilibrium of material. In addition, by small changes to the used apparatus volume defects can be overcome, obtaining a fiber whose vitreous structure better approaches an ideal.